The invention relates generally to the field of temperature compensating circuits for crystal oscillators. In particular, the invention concerns a compensating circuit which creates a control voltage that is applied across a varactor diode for maintaining the frequency of a crystal oscillator at a substantially constant value as the temperature of the oscillator is varied.
Oscillators which have a frequency determining crystal are commonly used to provide a stable output frequency. However, the crystals used in such oscillators are temperature sensitive, and therefore temperature compensating apparatus is normally required to maintain an extremely stable oscillator output frequency. Two basic techniques are used for temperature stabilizing the crystal oscillator frequency. One method is to enclose the oscillator within an oven and thereby maintain the crystal at a constant temperature. This requires a large amount of space and consumes a substantial amount of power. Another method, which is the method generally adopted by the present invention, is to generate a temperature varying voltage and apply it across a voltage variable capacitor (varactor diode) to control the resonant frequency of the crystal oscillator.
In most oscillators, the AT cut crystal is commonly used and it has a generally cubic frequency versus temperature characteristic having an inflection point at approximately +26.5.degree.C. The exact temperature characteristics of individual AT cut crystals are quite variable and depend on how the crystal was made. Thus in order to perfectly compensate an oscillator using an AT crystal, the voltage applied to the varactor diode should have a temperature variation which is substantially similar to that of the particular crystal being used.
Some prior circuits have created a cubic law temperature varying voltage by twice multiplying a linearly varying voltage, but such systems are extremely complex and cannot be adequately and easily adjusted to fit the compensating voltage versus temperature curve which is required by any one particular crystal oscillator.
Another common method which partially compensates a crystal oscillator using AT cut crystals uses hot and cold temperature range networks to produce non-linear temperature variations in a control voltage above and below two predetermined temperatures, while applying a constant control voltage in a middle temperature range. In addition, temperature sensitive capacitors are also used in the crsytal oscillator to minimize the effect of the substantially linear frequency versus temperature variation of the crystal that exists in the middle temperature range. Such circuits only partially compensate the resonant crystal. They are also not suitable for applications in which the crystal is operated in an overtone mode of oscillation, since temperature sensitive capacitors are then generally unable to adequately compensate for the linear variation in the middle temperature range.
Still another method of producing a temperature compensating control voltage, is to use a thermistor and a series of zener diodes having different breakdown voltages to create a "piecewise" non-linear voltage which is adjusted to fit a desired curve. A disadvantage of this system is that any adjustment of an individual piecewise non-linear section, will affect a number of other sections and require their readjustment which will in turn require other subsequent adjustments. An additional disadvantage is that many components are needed to create an adequately fitting composite curve. This composite curve has abrupt (step) changes in slope for every piecewise section and therefore perfect compensation is never feasible. Also, the design of the compensating network is difficult because zener diodes are available with only certain discrete breakdown voltages.
Thus all other prior systems either only partially temperature compensate the crystal oscillator by applying a control voltage across a varactor diode and/or dependently create a temperature varying control voltage which cannot be readily adjusted to exactly compensate any crystal oscillator using a particular AT crystal.